You found my suggestion to put the Next button, that is meant as a Skip button during Quiz, under the Clear button, which has also the advantage that there cannot be a confusion between the Submit and the Next button Indeed, it was certainly also mentioned that it is supposed you don't allow Backwards movement and hide the playbar in that case.
http://blog.lilybiri.com/question-question-slides-in-captivate
The buttons on question slides cannot be controlled like other buttons. They are embedded in the question slide. If you want total control, you have to create custom question slides (lot of work).
Another approach with the default question slides is to check the Required option 'Answer All' in the Quiz Preferences, Settings. Or you can use Submit All, which allows backward movement and will pop up a warning if not all questions are answered, but will alow to submit even in that case.
And you could try, if you feel comfortable with advanced actions, to create a conditional action On enter for each slide, that shows a shape button that will be Next button in case of backwards movement..
Lilybiri